Composite and abrasive plastic materials do not machine the way metals do. Metal cuts — the chip shears off cleanly, the edge engages and releases. Composites and filled plastics grind. The reinforcing fibers or abrasive filler particles act like distributed cutting media against your drill geometry, wearing the edge continuously rather than discretely. If you are approaching these materials with the same tooling strategy you use for aluminum or mild steel, you are wearing out bits far faster than necessary and probably not getting clean holes either.
Abrasion Wear vs. Cutting Wear
Standard drill wear in metals is mostly cutting-edge wear — the edge rounds off from repeated shear contact, relief faces wear back, and the chisel edge broadens. This wear is gradual and somewhat forgiving. You can push through several warning signs before hole quality degrades enough to matter.
Abrasion wear in composites is different in character. The glass fibers in fiberglass and the carbon fibers in CFRP are extremely hard — glass runs around 5.5 on the Mohs scale, carbon fiber reinforcement is comparable. They act like fine abrasive particles distributed throughout the matrix. Every revolution of your drill is running those edges against that abrasive. The outer corners of the cutting lips — already the most vulnerable section of a standard drill — wear rapidly. Lip relief erodes. The cutting edges go from sharp to dull to rounded in far fewer holes than you would see in most metals.
What this means practically: your standard HSS jobber drill that gets resharpened and runs 500 holes in mild steel might go dull in 50 holes in fiberglass. That is not an exaggeration for production work.
What Standard Geometry Gets Wrong
The 118-degree included point angle on a standard jobber drill is designed for metals where the chip shears cleanly. It is too aggressive for composites in several ways. The steep rake angle that cuts metal efficiently tends to push composite fibers laterally before cutting them, causing delamination and fraying at the hole entry and exit. In carbon fiber especially, exit delamination is a quality and structural problem — those peeled layers cannot simply be ignored.
The tip geometry that works better for composites is more of a brad-point or modified brad-point style — a positive outer lip with a more acute included angle, often 90 to 100 degrees, that slices through fibers rather than pushing them. Purpose-made composite drill bits use these geometries. They still wear in abrasive materials, but the hole quality is better while they last, and the wear pattern is more manageable.
For fiberglass in particular, a 90-degree point with a flat or brad-point tip geometry is a significant improvement over a standard jobber. The flatter angle reduces the tendency to push the laminate down before cutting through it, and the outer cutting lips engage cleanly before the center of the hole is being disturbed.
Fiberglass Specifics
Fiberglass — whether woven cloth laminates, chopped mat, or filament-wound — is about as hard on tooling as anything you will drill in a production environment. The glass content varies, but even a moderate glass loading will chew through standard HSS quickly. Heat is also a factor: the resin matrix softens when hot, can melt onto your drill geometry, and the resulting smearing makes hole quality worse while accelerating mechanical wear.
Run fiberglass dry if at all possible. Coolant in fiberglass creates a slurry of glass particles and wet resin that is worse for tooling and harder to manage than dry cutting. Use compressed air to clear chips. Run speeds fast enough to cut cleanly — unlike titanium, you want to cut through the material quickly and limit dwell time — but not so fast that resin melt becomes a problem. For most fiberglass work with a 1/4" drill, something in the 200-300 SFM range is a reasonable starting point.
Carbon Fiber (CFRP) Specifics
Carbon fiber reinforced polymer is more dimensionally stable and less prone to resin melt than fiberglass, but the carbon fibers are aggressive abrasives. Carbide is the standard for production CFRP work for this reason — HSS simply does not have the wear resistance to run efficiently. If you are doing occasional CFRP work or prototype quantities, HSS will work, but set expectations accordingly.
Delamination control in CFRP requires sharp geometry. A dull bit is worse than a dull bit in any other material — the blunt edge pushes fibers rather than cutting them, and delamination at entry and exit becomes severe. Frequent edge changes or resharpening is not optional if hole quality matters. For structural CFRP work, inspect the entry and exit of each hole. If you are seeing fiber pullout or visible delamination rings, the bit needs attention.
Backup material under the exit face — a sacrificial piece of scrap — dramatically reduces exit delamination regardless of drill geometry. This is standard practice in aerospace CFRP work and worth doing even in lower-stakes applications.
Abrasive Filled Plastics
Glass-filled nylons, glass-filled PTFE, mineral-filled engineering plastics, and fiber-reinforced thermoplastics all cause abrasion wear that is out of proportion to their hardness. A 30% glass-filled nylon is soft enough to drill easily in one sense — the matrix cuts without much force — but the glass filler is doing constant abrasive work on your geometry.
For these materials, the practical advice is the same as for fiberglass but with more attention to chip clearing. Plastics can pack in flutes and generate heat from friction rather than cutting. Peck drilling and periodic retraction to clear the flutes matters more in deep holes through filled plastics.
Does Mail-In Resharpening Make Sense for Composite Drilling?
The honest answer is: it depends on the geometry you need. If you are using standard jobber drills in composites and getting poor life, resharpening those drills to standard geometry returns them to original sharpness but does not fix the geometry problem. You will get more holes per edge than you would with a dull bit, but you are still using a geometry that is not optimized for composites.
Where mail-in resharpening makes clear sense for composite work is on larger diameter drills — 3/8" and up — where the cost of new bits is significant, and the geometry can be restored to a sharp, consistent edge. For shops drilling occasional composites alongside regular metal work, having a dedicated set of larger HSS bits resharpened to composite-appropriate geometry is a cost-effective approach compared to buying carbide equivalents. MachinistPost can restore lip geometry on worn HSS drills used in composite work (we sharpen HSS only, not carbide) — if you are sending in bits from mixed-material work, note on the order form what material they were running so the grind can be matched appropriately.
For small diameter drills under 1/4", the economics typically favor new bits in abrasive materials. The bits are cheap enough and the wear rapid enough that replacement makes more sense than resharpening.
Dust and Chip Management
This section is not about drill life but it matters enough to include. Fiberglass and carbon fiber dust are serious health hazards. Glass fiber particles are a respiratory irritant. Carbon fiber particles are finer and stay suspended longer. Neither should be breathed. Use a vacuum at the cutting zone, wear an N95 minimum, and take the dust situation seriously. Shops that drill composites regularly without proper extraction are creating a long-term health problem for their people.